Liquid crystalline materials (LCMs) exhibit phases having some of the anisotropic properties of solids (orientational or positional order) yet are free flowing liquids. LCMs have been utilized in various applications including displays, thermometers, optical switches, wavelength tunable filters, lasing media, optical polarizers, and sensing materials. Many devices incorporate materials exhibiting a nematic liquid crystal phase, where molecules possess orientational order but lack any positional order. When nematic phases are formed on isotropic substrates, such as glass, many nematic domains form where the vectors that describe the orientational order (director) vary in direction across the mesophase. (FIG. 1) However, typically, devices that utilize liquid crystalline phases require substantially uniform alignment of the liquid crystal phase, as shown in FIG. 1.
In order to induce alignment, a technique known as rubbing was developed involving a thin polymer film rubbed in one direction using a cloth to generate microscopic and molecular anisotropies on the surface of the substrate. Rubbing is a major industrial technique for producing aligned LCMs. However, there are a number of problems associated with the rubbing process such as: (i) generation of static charges in the device, (ii) incorporation of dust particles or other impurities on the surface, (iii) the inability to create patterns, and (iv) it can serve as a bottleneck in the production of LCMs.
In order to solve the problems with rubbing, there has been considerable interest in developing non-contact alignment processes that utilize light to align liquid crystals (e.g., photoalignment). The general approach is to coat a substrate with a photoresponsive polymeric film, and through polarized irradiation, generate anisotropy at the liquid crystal-polymer interface. The photoresponsive polymer films typically rely on simple photochemical transformations such as cis/trans isomerizations and cycloaddition reactions. Some commonly used transformations for photoalignment include cis-trans isomerizations of olefins, fragmentation reactions (e.g., of polymers or dimers into monomers), cycloadditions (e.g., [2+2] cycloadditions). However, such reactions generally suffer from at least one of the following limitations: (i) the transformation is thermally reversible, (ii) more than one pathway is available, (iii) the process is inefficient, (iv) the process promotes only weak alignment forces on the liquid crystal, and/or (v) extended irradiation destroys the alignment.